rt2x00: rt2500usb: implement queue_init callback

The generic rt2x00 code has been changed to allow the
drivers toimplement dynamic data_queue initialization.

Remove the static data queue descriptor structures
and implement the queue_init callback instead.

Compile tested only.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>